Light-receiving method of an avalanche photodiode and a bias control circuit of the same

ABSTRACT

The present invention provides a method to maintain a multiplication factor of an avalanche photodiode independent on temperatures without additional devices. The light-receiving apparatus of the invention includes an avalanche photodiode (APD), a dividing circuit, and a bias supplying circuit. The APD has a first region, where a significant multiplication factor appears, and a second region without any multiplication factor. The dividing circuit extracts a second signal

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light-receiving method of anavalanche photodiode (APD) and a bias control circuit for the APD.

2. Related Prior Art

The APD, which uses a physical phenomenon of the avalanche breakdown ofthe semiconductor p-n junction at a high reverse bias voltage, has amultiplication factor greater than unity. The multiplication factormeans that how many electrical carriers can be generated by a signalphoton. Therefore, the APD can generate a large photo current from aweak optical signal. The design or the specification of the circuitconnected to the APD strongly depends on how large the multiplicationfactor thereof is set. The PIN-PD, which is a semiconductorlight-receiving device similar to the APD, generally has amultiplication factor smaller than unity because the PIN-PD shows noavalanche breakdown phenomenon.

When a large multiplication factor is set by applying the high reversebias voltage to the APD, a noise involved in the photo current outputfrom the APD will also increase. On the contrary, a small multiplicationfactor leads the optical sensitivity of the APD insufficient to apresetting specification. The Japanese patent application published asH09-321710 has disclosed that the bias voltage to the APD is controlledto maximize the signal-to-noise ratio (SNR) thereof. The method uses twofilters, one of which extracts the signal component and the otherextracts the noise component. The SNR is calculated from thus extractedsignal and noise components, and the bias voltage is applied to the APDso as to maximize the calculated SNR,

Japanese patent application published as 2000-244418 has disclosed amethod for controlling the bias voltage to the APD based on an opticalinput level and ambient temperatures. In this disclosure, a PIN-PDprovided in addition to the APD receives the input light, and the biasvoltage applied to the APD is controlled by the reference signalgenerated by the PIN-PD. This patent application has also disclosed thatthe bias voltage applied to the APD is adjusted based on the ambienttemperature.

These prior applications have disclosed that the bias voltage applied tothe APD, namely the multiplication factor thereof, may be adjusted asthe change of the optical input level of the temperature. However, asshown in FIG. 7, the photo current output from the APD changes evenunder the constant bias voltage. Especially, in a region of the biasvoltage from 30V to 60V where a significant multiplication factor isobtained, the output photo current, i.e. the multiplication factor,widely changes as the temperature varies.

The PIN-PD has a quite smaller temperature dependence of themultiplication factor compared to that of the APD, and the magnitudethereof is nearly unity. Accordingly, by using the PIN-PD as a monitordevice for the input light and controlling the bias voltage applied tothe APD based on the output from the PIN-PD, the multiplication factorof the APD, especially a drift for the temperature, may be suppressed.However, to prepare the PIN-PD independently on the APD becomes anapparatus to be complex, and to adjust the light-receiving conditionbetween the APD and thus prepared PIN-PD may be a troublesome procedure.Thus, the independent PIN-PD on the APD may not appropriately controlthe bias voltage to the APD.

SUMMARY OF THE INVENTION

One object of the present invention is to provide an optical apparatus,in which a monitor signal with small temperature dependence may beobtained by providing no additional devices except the APD, and toprovide a method for controlling the bias voltage to the APD based onthus provided monitor signal.

According to one aspect of the present invention, an optical apparatusfor receiving signal light is provided. The optical apparatus includesan avalanche photodiode (APD) and a bias control circuit. The APD has alight-receiving surface that includes a first region and a secondregion. The APD outputs a photo current that includes a signal componentgenerated in the first region and a monitor component generated in thesecond region.

The bias control circuit outputs a bias voltage to the APD and includesa high-pass-filter (HPF), a variable gain amplifier and a comparator.The HPF extracts the signal component from the photo current. Thevariable gain amplifier extracts the monitor component by comparing thephoto current with the signal component output from the HPF, andamplifying thus extracted monitor component by a preset gain. The biascontrol circuit controls the bias voltage applied to the APD such thatthe signal component is substantially equal to the amplified monitorcomponent.

The bias control circuit of the present invention may further include apeak hold circuit that holds a peak value of the extracted signalcomponent output from the HPF. The comparator may compare the peak valueprovided from the peak hold circuit with the amplified monitor componentoutput from the variable gain amplifier.

The bias control circuit may further-include a low-pass-filter (LPF)that smoothes the photo current. The variable gain amplifier may extractthe monitor component by comparing the smoothed photo current with theextracted signal component output from the HPF.

The bias control circuit may further include an extracting amplifierthat extracts the monitor component by comparing the photo current withthe signal component extracted by the HPF. The preset gain of thevariable gain amplifier is to be controlled based on the monitor signalextracted by the extracting amplifier.

Another aspect of the present invention is to provide an avalanche photodiode (APD) that outputs a photo current corresponding to receivedsignal light by applying a bias voltage. The APD of the presentinvention includes a first electrode, a semiconductor substrate, alight-sensitive layer, a highly doped layer, and a second electrode. Thesubstrate has a first conduction type and is provided on the firstelectrode. The light-sensitive layer has a first conduction type and isprovided on the substrate. The highly doped layer has a secondconduction type and is provided in a portion of the light-sensitivelayer. The second electrode is in contact with the highly doped layer.Further, the APD of the present invention has a first and secondregions. The bias voltage is applied mainly to the first region suchthat the first region, which includes the highly doped layer and has asignificant multiplication factor, generates a first photo currentcorresponding to a portion of the signal light being incident in thefirst region, and the second region, which has a multiplication factorof substantially unity, generates a second photo current correspondingto a portion of the signal light being incident in the second region.

Still further aspect of the present invention is to provide a method forcontrolling a multiplication factor of the APD. The APD has alight-receiving surface that includes first and second region andoutputs a photo current that includes a signal component and a monitorcomponent generated in the first and second regions, respectively. Themethod may comprise: (a) extracting the signal component from the photocurrent; (b) extracting the monitor component from the photo current bycomparing the photo current with the extracted signal component; (c)amplifying the extracted monitor component by a preset gain; and (d)comparing the amplified monitor component with the extracted signalcomponent The multiplication factor of the APD may be controlled suchthat the bias voltage applied to the APD is adjusted such that theextracted signal component is substantially equal to the amplifiedmonitor component.

The method may further include a step of, before the step (c),determining the preset gain based on the monitor component extractedfrom the photo current by comparing the photo current with the extractedsignal component.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a plan view showing an avalanche photodiode (APD) of thepresent invention; and FIG. 1B is a sectional view showing the APD ofthe present invention;

FIG. 2 shows a relation between the photo current generated by the APDshown in FIG. 1 and the region where the light is incident therein;

FIG. 3 shows a relation between the frequency response of the photocurrent detected in the first and second regions of the APD shown inFIG. 1;

FIG. 4 is a block diagram of the control circuit according to the firstembodiment of the present invention;

FIG. 5 is a block diagram of the control circuit according to the secondembodiment of the present invention;

FIG. 6A shows a first configuration of the APD by which the presentinvention is carried out, and FIG. 6B shows a second configuration ofthe present invention; and

FIG. 7 shows a typical relationship between the photo current and theapplied bias voltage of the APD at various temperatures.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS First Embodiment

Next, preferred embodiments of the present invention will be describedas referring to accompanying drawings. FIG. 1A and FIG. 1B are a planand a sectional view, respectively, they are showing an avalanchephotodiode (APD) used in the present invention. The APD 1 includes aheavily doped substrate 2, an n-type active layer 3, a heavily dopedp-type diffusion layer 4, an insulating film 6, a p-electrode 7 with apad 7A, and an n-electrode 8. Further, the APD includes a firstsensitivity region P_(S) and a second sensitivity region P_(M) on aprimary surface thereof.

As shown in FIG. 1A, the first region P_(S), which has a multiplicationfunction, is formed in a restricted center region of the surface. Thep-electrode 7 formed to surround the first region P_(S), extracts thepad 7A to which a bias voltage for the PD to be supplied between then-electrode 8 provided in a whole surface of the back surface of the APD1. The APD 1 is formed such that (1) the active layer 3 is grown on then⁺-type substrate 2, (2) the diffusion mask 5, which is made of SiO₂ andhas an diffusion opening, is formed on the active layer 2, and (3)acceptor dopants are diffused into the active layer 3 via the diffusionopening, which forms the p⁺-type diffusion layer 4.

The ring-shaped p-electrode 7 formed around the p⁺-diffusion layer 4 isextracted onto the insulating layer 5. Within the p-electrode 7 isprovided with a second insulating film 6 made of Si₃N₄, which has athickness to operates as an anti-reflection coating for a signalwavelength λ_(S). As shown in the sectional view of FIG. 1B, when thebias voltage is provided between the p-electrode 7 and the n-electrode8, this bias voltage is affected only to a portion just under the firstregion P_(S), not applied to portions out of the first region P_(S).

The insulating film 6 out of the p-electrode 7 is formed so as to betransparent for the wavelength λ_(m) of the signal light, and then-electrode 8 is formed in the whole back surface of the APD 1.Accordingly, incident light into the portion outside the p-electrodegenerates photo-currents, but the multiplication factor therein becomessmaller than unity, because the bias voltage is not affected to thisregion, thus no electric field is induced. In the present invention, theportion within the p-electrode and showing a significant multiplicationfactor will be called as the first region P_(S), while the portionoutside the p-electrode and showing no electric field and nomultiplication factor is called as the second region P_(M).

FIG. 2 shows a sensitivity of the APD along a line B-B intersecting bothregions of P_(S) and P_(M), which is shown in FIG. 1A. The vertical axisdenotes a magnitude of the photo current. Center portion of FIG. 2corresponds to the first region P_(S), while both sides of the centerportion correspond to the second region P_(M). When the light enters thefirst region P_(S), a large photo current will be obtained. On the otherhand, when the light enters the second region outside of the p-electrode7, the magnitude of the photo current becomes only ⅓ compared to that ofthe first region P_(S). Further, since the second region P_(M) is notaffected from the bias voltage, the temperature dependence of themultiplication factor therein also becomes small compared to that in thefirst region.

FIG. 3 show frequency responses of the multiplication factor attributedto the first and second regions. The first region is capable ofresponding to high frequency signals over 1 GHz. On the other hand, thatof the second region remarkably decreases in high frequencies inaddition to the magnitude thereof being unity at most. The response ofthe second region P_(M) is limited to regions below 0.5 GHz.

Therefore, two signals each generated in the first and second regionsP_(S) and P_(M), may be divided by passing a high-pass-filter, a cut-offfrequency of which is about 500 MHz. That is, a signal passing throughthe high-pass-filter only involves the signal attributed to the firstregion, while another signal not passing through the filter involvesboth signals. Therefore, by processing these two signals, one isoriginal and the other is pass-through the filter, two signalsoriginally attributed to first and second regions, P_(S) and P_(M),respectively, can be distinguished. Further, the bias voltage can becontrolled so as to maintain the multiplication factor or to maintainthe output of the APD independent of the optical input.

FIG. 4 and FIG. 5 show examples of the control circuit to maintain themultiplication factor of the APD. The control circuit includes an APD10, a pre-amplifier 11, which is often called as a trans-impedanceamplifier (TIA), a high-pass filter (HPF) 12, a low-pass filter (LPF)13, a limiting amplifier 14, a peak-hold circuit 15, an operationalamplifiers 16, 17 and 18, and a DC/DC converter 19.

The APD 10, as shown previously in FIG. 1, includes first and secondregions. The former shows a significant multiplication factor dependingon the bias voltage applied thereto, while the latter is not affected bythe bias voltage and shows no significant multiplicationcharacteristics. The signal output from the APD 10 includes componentsof the first signal I_(S), the second signal I_(M), and the third signalI_(NO). They correspond to the optical signal detected in the firstregion P_(S), the other optical signal detected in the second regionP_(M), and the noise that is common in the first region P_(S) and thesecond region P_(M), respectively. The signal output from the APD 10 isinput into the trans-impedance amplifier (TIA) 11. The noise componentI_(NO) includes a noise intrinsically involved in the optical signalitself, that generated at the conversion from the optical to theelectrical data at the APD 10, that involved in the bias supply, andthose derived from the whole other reasons.

The TIA 11 converts the current signal including I_(S), I_(M) and I_(NO)into a corresponding voltage signal. The high-pass-filter (HPF) 12extracts the signal I_(S) and the noise I_(NO), cutting the monitorsignal I_(M). On the other hand, the low-pass-filter (LPF) 13 stillincludes all signal components of I_(S), I_(M), and I_(NO).

The first signal (I_(S)+I_(NO)), passing the HPF 12, is amplified by andoutput from the limiting amplifier 14. Further, the magnitude of thefirst signal may be obtained by passing the peak hold 15, such as arectification circuit including a diode and a capacitor as a load of thediode. On the other hand, the second signal (I_(S)+I_(M)+I_(NO)) passingthe LPF 13 may be automatically obtained in its magnitude by setting thecut-off frequency of the LPF 13 to be quite small frequency. These firstand second signals are compared and amplified in their difference by thevariable gain amplifier (U1) 16.

Thus, the variable gain amplifier 16 may output the component of onlythe monitor signal I_(M) multiplied by the gain G thereof. The amplifiedmonitor signal I_(M)×G and the first signal (I_(S)+I_(NO)) aredifferentiated by another amplifier (U2) 17. Finally, theDC/DC-converter 19 outputs the bias voltage to the APD 10 such thatthese amplified monitor signal I_(M)×G is equal to the first signalI_(S)+I_(NO).

This closed loop operation is carried out such that the first signal,which corresponds to the light detected in the first region P_(S) wherethe significant multiplication factor is affected, is equalized to themonitor signal compensated by the gain G, which corresponds to the lightdetected in the second region P_(M) where the bias voltage is notaffected and does not show the significant multiplication factor. Sincethe monitor signal does not contain the components, the magnitude ofwhich is not affected to temperatures, the multiplication factor of theAPD 10 can be kept substantially constant even when the temperaturethereof changes.

Second Embodiment

The bias voltage for the APD is 10 conventionally configured such that,when the optical input becomes large, the multiplication factor mayautomatically decrease and reduce the bias current. However, the circuitshown in FIG. 4 controls the bias voltage to the APD 10 such that thefirst signal containing the I_(S) and I_(NO) is equal to the monitorsignal I_(M) multiplied by the gain G. The magnitude of the lightdetected at the first region P_(S) is proportional to that detected atthe second region P_(M). as far as the beam spot, the shape and thelocation on the APD 10, does not change. Therefore, the increase of thesignal I_(S) corresponding to the light detected by the first regionmeans that the signal I_(M) corresponding to the light detected by thesecond region becomes large. Therefore, the APD 10 may be broken by thephoto current generated by it self at the condition that a large opticalenters. Further, when the photo current generated by the APD 10 becomeslarge, the circuit connected to the APD may saturate.

One solution to solve such situation that the large optical signalenters the APD 10 is shown in FIG. 5. The bias voltage to the APD 10 inthe circuit of FIG. 5 may be changed as the optical signal increases.The APD 10 has the same configuration with that of shown in FIG. 4,namely, which generates the current signal I_(S) corresponding to thelight received in the first region P_(S), another current signal I_(M)corresponding to the light received in the second region P_(M), and thenoise component I_(NO) commonly involved in both I_(S) and I_(M). Thesignal including these components is input to the TIA 11.

The output of the TIA 11 is divided into two signals, one of whichcontains I_(S)+I_(NO) after passing the HPF 12 and the peak hold 15, theother of which, containing I_(S), I_(M), and I_(NO), is input both theamplifier (U0) 18 and the amplifier (U1) 16 after passing the LPF 13.The output of the amplifier U1, same with that shown in FIG. 4,generates the second signal, the monitor signal multiplied by the gainof the amplifier U1 (G×I_(M)). The difference between the first signal(I_(S)+I_(NO)) and the second signal (G×I_(M)) may be detected by theamplifier U2, and the bias voltage V_(BIAS) to the APD 10 is controlledby the DC/DC-converter 19 such that the difference of the first andsecond signals becomes zero.

However, the circuit shown in FIG. 5, when the monitor signal I_(M),which is generates by the amplifier U0 by comparing the first signal(I_(S)+I_(NO)) and the second signal (I_(S)+I_(M)+I_(NO)), becomeslarge, the gain G of the amplifier U1 may decrease. Contrary to theabove situation, when the monitor signal I_(M) becomes small, the gain Gof the amplifier U1 becomes large, thereby keeping the output, which isthe monitor signal multiplied by the gain thereof (G×I_(M)), of thevariable gain amplifier U1 constant. Thus, by the configuration shown inFIG. 5, even when the temperature of the APD 10 changes and themagnitude of the optical input changes, the output of the APD 10 can bemaintained to the predetermined value.

FIG. 6A shows a configuration for the APD to carry out the presentinvention, and FIG. 6B shows another configuration. The primary surface,the light-sensitive surface, of the APD 20 includes the first regionP_(S), where the significant multiplication factor is appeared by theapplication of the bias voltage, and the second region P_(M), where nomultiplication factor is appeared. Conventionally, the light S isfocused by the condenser lens 21 so as to be incident only in the firstregion P_(S).

In the present invention, as shown in FIG. 6A, the lens 21 may expandthe beam spot of the light S such that not only the first region P_(S)but also the second region P_(M) may receive the portion of the light S.The greater part of the light S is detected by the first region P_(S), aportion of the light S may be detected in the second region P_(M). Thephoto current I_(M) derived from the second region P_(M) can be soutilized in the present invention that shown in FIG. 4 and FIG. 5.

FIG. 6B shows another configuration, by which the present invention maycarry out. A half mirror 22 divides the light S condensed by the lens 21into two beams. One of beams is incident in the first region P_(S) andgenerates the current I_(S), while the other beam is incident in thesecond region P_(M) and generates the monitor signal I_(M).

Although illustrative embodiments of the present invention have beendescribed herein with reference to the accompanying drawings, it is tobe understood that the invention is not limited to those preciseembodiments, and that various other changes and modifications may beeffected therein by one skilled in the art without departing from thescope or spirit of the present invention.

1. An optical apparatus for receiving signal light, comprising: anavalanche photodiode having a light-receiving surface including a firstregion and a second region, said avalanche photodiode outputting a photocurrent including a signal component generated in said first region anda monitor component generated in said second region; a bias controlcircuit for outputting a bias voltage to be provided to said avalanchephotodiode, said bias control circuit comprising, a high-pass-filter forextracting said signal component from said photo current output fromsaid avalanche photodiode, a variable gain amplifier having a presetgain, said variable gain amplifier extracting said monitor componentfrom said photo current by comparing said photo current with said signalcomponent output from said high-pass-filter, and amplifying saidextracted monitor component by said preset gain; and a comparator forcomparing said signal component and said amplified monitor component,wherein said bias voltage applied to said avalanche photodiode iscontrolled such that said signal component is substantially equal tosaid amplified monitor component.
 2. The optical apparatus according toclaim 1, wherein said bias control circuit further comprises a peak holdcircuit for holding a peak value of said extracted signal component,said comparator comparing said peak value with said amplified monitorcomponent, and wherein said bias voltage applied to said avalanchephotodiode is controlled such that said peak value output from said peakhold circuit is equal to said amplified monitor component.
 3. Theoptical apparatus according to claim 1, wherein said bias controlcircuit further comprises a low-pass filter for smoothing said monitorsignal and for outputting a smoothed monitor signal, said variable gainamplifier extracting said gained monitor signal from said signalcomponent and said smoothed monitor signal.
 4. The optical apparatusaccording to claim 1, wherein said bias control circuit hither comprisesan extracting amplifier for extracting said monitor component bycomparing said photo current with said signal component extracted bysaid high-pass-filter, said preset gain of said variable gain amplifierbeing controlled based on said monitor component extracted by saidextracting amplifier. 5-6. (canceled)
 7. A method for controlling amultiplication factor of an avalanche photodiode having alight-receiving surface including a first region and a second region,said avalanche photodiode outputting a photo current including a signalcomponent generated in said first region and a monitor componentgenerated in said second region, said method comprising steps of (a)extracting said signal component from said photo current; (b) extractingsaid monitor component from said photo current by comparing said photocurrent with said extracted signal component; (c) amplifying saidextracted monitor component by a preset gain; and (d) comparing saidamplified monitor component with said extracted signal component,wherein said bias voltage applied to said avalanche photodiode iscontrolled such that said extracted signal component is substantiallyequal to said amplified monitor component.
 8. The method according toclaim 7, further comprising a step of, before step (c), determining saidpreset gain based on said monitor component extracted from said photocurrent by comparing said photo current with said extracted signalcomponent.